1. Field of the Invention
The present invention relates to a sensor for the efficient inspection and measurement of objects such as semiconductor wafers upon which dielectric and conducting films are deposited.
2. Description of the Related Art
In the process of manufacturing integrated circuits, films are deposited on semiconductor wafers, etched, and overlaid by additional films to create the integrated circuits. The films may be either conducting metal films or dielectric films. Quality and thickness of a particular film layer is critical to the manufacturing process, and it is of particular interest to measure the thickness of a film layer quickly and accurately. Two types of measurement techniques are commonly used to measure thickness. Dielectric films are transparent and thickness may be measured using optical reflectance. Conducting metal film thickness may be measured using eddy current sensing techniques. Further, if the conducting metal film is thin enough, it is transparent or semi-transparent which allows the thickness of the film also to be measured using optical reflectance. Both techniques are known and used in the industry.
Optical reflectance may be used to determine characteristics such as thickness, index of refraction and index of extinction of transparent and semi-transparent films. Metal films with a thickness greater than a few tens of nanometers tend to be optically opaque to light having wavelengths between 200 and 1100 nanometers. As a result, optical reflectance measurements are ineffective to determine thickness for such films. Eddy current sensing may be used to measure sheet resistance, which is correlative of such electrically conducting film thickness. Thus eddy current sensing generally may be used to measure thick conducting films and optical reflectance sensing generally may be used to measure thin conducting films that are transparent or semi-transparent.
Eddy current sensing techniques do not work for measuring thickness of films which do not conduct electricity sufficiently such as dielectric films. As a result, it is necessary to rely on optical reflectance techniques to measure dielectric film thickness.
The optical reflectance of the conducting metal film can vary with surface oxidation and grain size. After a conducting metal film is deposited on a semiconductor wafer, an annealing step is carried out to stabilize the film, to improve the conductivity of the metal film by enhancing the size of the metal grains, and with the proper reduction agent, to clean oxide from the film. This annealing process changes the grain size of the conducting metal film which changes the optical reflectance characteristics and the sheet resistance. The change in film reflectance after annealing can be correlated to both oxide removal and grain size variations.
Moreover, eddy current sensing alone cannot distinguish whether sheet resistance varies because of film thickness or because of grain size change caused by the annealing process. Therefore, both optical reflectance measurements and eddy current sensing measurements are required to adequately determine thickness of a conducting metal film, and it is advantageous to take both measurements during a single inspection. Optical reflectance measurements can compensate for variations in sheet resistance due to grain size variations thereby allowing eddy current sensing measurements to correlate accurately with film thickness.
Both the optical reflectance and the eddy current sensing methods of inspecting conducting film are well known to skilled practitioners of the art. However, each method has been employed separately and apart from the other. In view of the foregoing, it would be desirable to be able to rely on different types of information derived from a combination of both optical reflectance and eddy current sensing measurements.